The invention relates to apparatus, systems and methods for controlling or adjusting the traction of a downhole tractor in a borehole.
In the petroleum exploration and production industries, downhole tractors are often used to convey tools and other devices into boreholes. However, downhole tractors may be used for any desired purpose. As used throughout this patent, the terms “tractor”, “downhole tractor” and variations thereof means a powered device of any form, configuration and components capable of crawling or moving within a borehole. The term “borehole” and variations thereof means and includes any underground hole, passageway or area. An “open borehole” is a borehole that does not have a casing. A “non-vertical borehole” is a borehole that is at least partially not vertically oriented, such as a horizontal or deviated well.
Typically, the movement of the tractor is enabled by friction-generated traction between one or more component associated with the tractor, referred to herein as the “drive unit(s),” and the borehole wall. In such instances, a normal force is usually applied to the drive unit to press it against the borehole wall.
For a tractor to achieve or maintain movement within a borehole, the drive unit cannot completely slip relative to the borehole wall, so that the traction force (FT)≦μFN, where μ is the friction coefficient between the drive unit and the borehole wall and FN is the normal force. Also, the drive unit must provide enough traction force to overcome drag or resistance (FR) on the drive unit, such as may be caused by the conveyed tool(s) and delivery cable, so that FT≧FR.
Any number of other factors (referred to throughout this patent as “disturbance factors”) may affect the amount of traction necessary to move the tractor within the borehole in any particular situation and environment of operation. For example, when the borehole wall possesses an irregular surface, the amount of traction necessary for movement and/or the coefficient of friction may change as the borehole surface navigated by the tractor changes. A few other examples of disturbance factors that may affect the tractor's resistance to motion are changes in the inclination of the borehole, diameter of the borehole, surface of the borehole, borehole wall properties, increasing cable drag (when a cable is used), debris in the borehole and borehole fluid properties.
When the amount of traction needed for the tractor to move or continue moving in the borehole changes, the normal force on the drive unit(s) must be adjusted. Otherwise, the tractor may experience excessive slippage. Hence, in order to keep FT≦μFN, the normal force FN has to be adjusted. The normal force may also need to be adjusted when it is desired to prevent power overload or unnecessary excessive normal force. Thus, although not essential for tractor operations (or the present invention), an ideal value for the normal force is FN=FT/μ, particularly when the tractor is moving in an open, non-vertical or highly deviated borehole.
If the borehole conditions change infrequently and there are no substantial tractor disturbance factors, such as may exist in a “cased” borehole, the normal force may be effectively adjusted by an operator sending commands to the tractor from the surface using existing technology. However, when the amount of needed traction changes often, such as in an open borehole or because of the existence of disturbance factors, the operator is unlikely to react sufficiently, often or quickly enough, resulting in excessive slippage and, thus, poor tractor performance, and/or excessive power to the drive units. Examples of existing downhole tractor technology not believed to provide sufficient or efficient traction control in such instances are disclosed in U.S. Pat. No. 6,089,323 issued on Jul. 18, 2000 to Newman et al. and U.S. Pat. No. 5,184,676 issued on Feb. 9, 1993 to Graham et al. Examples of existing traction control technology for entirely different applications not involving downhole tractors are U.S. Pat. No. 6,387,009B1 to Haka and issued on May 14, 2002 and German Patent DE 19,718,515 to Bellgardt and issued on Mar. 26, 1998. Each of the above-referenced patents is hereby incorporated by reference herein in its entirety.
Thus, there remains a need for methods, apparatus and/or systems that are useful with downhole tractors and have one or more of the following attributes, capabilities or features: adjusting the normal force on one or more drive unit continuously, automatically, without human intervention, on a real-time basis, or any combination thereof; optimizing the traction of the drive unit(s) in the borehole by adjusting or controlling the normal force; applying as much normal force as necessary to reduce slippage and as little normal force as necessary to minimize waste of available power; adjusting the normal force as quickly as possible without the necessity of human involvement; reacting to or dealing with typical disturbance factors by adjusting the normal force on the drive unit(s); real-time adjustment of normal forces on the drive unit(s) to maintain or cause movement of the tractor in the borehole; allowing the tractor to achieve continuous motion, as may be desired or required in downhole data logging applications, at the lowest effective normal force; preventing excessive or unnecessary wear on components, loss of energy and casing or formation damage caused by excessive normal forces.